The invention relates to a heterodyne receiver system incorporating a harmonic mixer, and more particularly to such system which is adapted to receive a sum or difference conversion with a specified order of harmonic automatically.
When determining the frequency of an input signal by counting it in a counter, the operational limit of flipflops involved imposes a practical limit on the order of 1000 MHz. A signal of higher frequency can not be directly counted for the determination of the frequency. Where it is desired to determine a frequency in excess of 1000 MHz by means of a counter, a crystal oscillator is used to produce harmonics, one of which is selected by a tuning circuit for the purpose of the frequency conversion of the input signal, with the converted signal of lower frequency being supplied to the counter in order to determine the frequency of the input signal from the count thereof. Generally, the frequency of the input signal is unknown, rendering it impossible to choose any particular harmonic to be supplied to the frequency conversion circuit. Consequently, the tuning frequency of the tuning circuit is varied from a lower order to a higher order harmonic to provide a frequency output which may be used in a sequential operation, which requires an increased time length for the determination of the frequency. Another proposal has been to supply the input signal and a local oscillator signal to a harmonic mixer so that the difference between the input signal frequency and the frequency of the local oscillator or a harmonic thereof may be obtained. As understood, when representing the frequency of the input signal by Fs, the frequency of the local oscillator signal by Fl and the frequency of the converted intermediate frequency by Fi, the following relationship applies: EQU Fs = nFl .+-. Fi
In this equation, n represents a positive integer which indicates the order of harmonic of the local oscillator signal, and n = 1 for the fundamental wave. Where the frequency determination is based on such relationship, it is necessary to know the particular value of n and the sign appearing in the equation, and for this purpose it has been the practice in the prior art to vary the frequency of the local oscillator signal Fl by a small frequency increment .DELTA.f to see the resulting variation of Fi or .+-.n .DELTA.f when an approximate expected value of the input signal frequency Fs is used in combination with several values of Fl and n. However, the procedure is complicated and in addition involves the possibility that the determination of n and the sign may be inaccurate.
It is therefore an object of the invention to provide a heterodyne receiver system incorporating a harmonic mixer in which the order of harmonic of the local oscillator signal and the sign in the mixing process may be determined in a simple and reliable manner.
It is another object of the invention to provide a heterodyne receiver system capable of receiving input signals over an extensive frequency range.
It is a further object of the invention to provide a heterodyne receiver system which automatically traps an input signal.
It is still another object of the invention to provide a heterodyne receiver system which permits the frequency of an input signal to be determined accurately and which operates in a stable manner.
It is a still further object of the invention to provide a heterodyne receiver system which is free from the influence of noises to trap an input signal and to determine the frequency thereof.
It is an additional object of the invention to provide a heterodyne receiver system in which the order of harmonic of the local signal is automatically switched.
It is a still additional object of the invention to provide a heterodyne receiver system which traps an input signal and determines its frequency by an arithmetic operation.
It is a further object of the invention to provide a heterodyne receiver system capable of displaying the input signal alone or in combination with accompanying signals.